In the process of making a web, such as a paper web or a web made of a plastic or plastic-like material, the web is moved through a dryer in which the web itself is dried or cured and/or a coating or other material which has been applied to, imbibed in, etc. the web material is to be dried or cured. Drying usually is referred to as the removing of moisture, such as water, solvent or another ingredient, e.g., by evaporation, from the web, coating, etc. Curing usually refers to the carrying out of a chemical reaction. However, drying and curing are used herein in the broadest sense; and for brevity the term drying will be used below inclusive also of curing. Also, for brevity reference herein to drying a web includes drying the web itself and/or a coating thereof.
The line speed at which emulsions, which are coated on a web, can be dried during a web manufacturing process, for example, is limited by how quickly water can be removed from the emulsion coating (drying flux) and the length of the dryer apparatus (dwell time of the web in the dryer). Line speed increases are limited by drying flux capacity of the dryer to dry the web without damaging the web. Line speed increases could be achieved if the dryer were lengthened to provide the required dwell time to obtain desired drying. There are similar considerations for curing a web. However, there are some disadvantages in making a dryer longer, such as the need to increase the number of zones in the dryer, which adds to the size, complexity, difficulty of control, and expense of the dryer, additional air handling equipment, and a longer web path in the dryer apparatus. Also, a longer unsupported span of web in a dryer, between dryers or drying zones, etc. can increase the risk of web breaks, snags, and/or other web handling problems; and, therefore, the risk of loss of material and time delays due to shutdowns are increased. It would be desirable to increase the capacity of a web dryer apparatus by running that apparatus at faster line speeds without increasing the dryer length. Accordingly, and consistent with the invention as is described in detail below, it would be desirable to provide an emulsion drying method and apparatus in which the drying flux capacity is increased so that emulsion coatings can be dried in a shorter dwell time in the dryer.
Some prior web dryers have used an air flotation technique to dry a web passing through the dryer. The air flotation oven dryer apparatus usually includes several air bars or nozzles located, respectively, facing opposite surfaces of the web. The web is moved along its path through the dryer, and heated air is blown toward the surfaces of the web by respective air bars. The air usually is heated to facilitate drying the web.
Blowing heated air toward the surfaces of a web, though, has been found to be relatively inefficient to dry a web. For example, the process of heating air is a relatively inefficient one, and the transferring of thermal energy to the web by air also is relatively inefficient. Also, the enthalpy of air is relatively low. However, it is desirable to heat the web to increase the drying flux and, therefore, the rate at which the material actually dries.
Several techniques have been used in the past to try to improve the drying flux and, therefore, to reduce the time required to dry a web. One technique was to design the air bars to direct air flow toward the web in a manner that creates an air foil effect to increase the wiping of the flowing air fluid against the web. Another technique was to direct the air flow from the air bars toward the web in several directions in order to create a somewhat turbulent flow at the web to increase the wiping of the air against the web and the transfer of thermal energy to the web. The air bars usually had to be relatively close to each other to get sufficient thermal energy transfer for drying, and the air bars themselves were relatively narrow in length dimension (direction of belt travel) to concentrate hot air toward/at the web without losing heat to the surrounding environment. The larger the number of air bars, though, the more expensive is such a prior air floatation dryer, and the more distortions are applied to the web, which possibly could cause damage to the web. Also, when the air bars are spaced more closely, the air flow is limited because there must be sufficient space to remove the exhaust air. Still further, with the air bars positioned close to each other, there may not be adequate room to locate electrodes for developing and applying RF field to the web.
Another disadvantage to the drying of a coating, such as an emulsion, on a web using the air flotation oven technique is that the coating surface tends to dry faster and to become hotter than the subsurface coating material, and the dry surface may become fused and/or difficult for subsurface moisture to penetrate and to escape to the external environment. Therefore, careful consideration must be given to controlling drying to take into account the moisture concentration profile in the coating material to achieve drying of the entire coating, not just the surface portion thereof. Such consideration may result in the reduction of the temperature of the air directed to the web, but the reduced temperature results in a smaller drying flux and reduced drying rate, which can slow the drying process or can require an increase in the path length of the web in the dryer.
Another technique for drying a coating on a paper web includes the directing of a stray field of radio frequency (hereafter abbreviated "RF") electromagnetic energy provided, for example, at from about 10 MHz to about 100 MHz to the web. Stray field electrodes are used to provide the stray field which heats the coating to cause drying. The web is supported relative to the electrodes by a flow of hot air which also removes steam clouds produced by the high-frequency RF energy stray field drying process. The air flow is provided via air bars which also may serve as electrodes to provide the RF stray field. However, a problem that can occur using such stray field drying process is blistering of the coating, which can occur when the coating becomes too hot while drying as it is exposed to the high-frequency electromagnetic energy and hot air. A web with a blistered coating usually is an unacceptable product. It would be desirable to use RF drying while avoiding such blistering or other heat damage to a web.
Blistering is one example of a defect caused in the coating during drying. Blistering may occur for several reasons. For example, if the temperature of the coating is raised too high or too fast, blistering may occur; or it may occur due to the formation of a skin on the coating which blocks release of subsurface moisture. It would be desirable to dry a web while minimizing defects, such as defects in the coating, e.g., blistering, and especially to effect such relatively defect-free drying at a relatively fast rate.
The invention is described below by way of example with respect to the drying of an emulsion type of coating on a paper web. In the drying process moisture, e.g., water, contained in the emulsion is removed from the emulsion. The result may be substantially all moisture being removed or only some of the moisture being removed, depending on the product. It will be appreciated that the moisture also may be removed from a coating that is other than an emulsion and that the moisture may be removed from the web itself. The coating may be on one or both surfaces of the web or the coating may be imbibed or otherwise in a sense absorbed in or carried by the web. In one example the web is paper, but it will be appreciated that the web may be of another material, such as a plastic or plastic-like material. The ingredient removed during the drying process may be a material other than or in addition to water. One example is a solvent. Another example is a carrier fluid. Also, the invention may be used to cure a material rather than or in addition to the drying of the material.
The invention may be used to provide air flow or the flow of some other fluid with respect to the web. The other fluid may be a gas or a liquid, depending on circumstances, such as characteristics of the web and/or coating, whether the gas is to participate in a chemical reaction, such as part of the curing process, etc. For brevity, though, the fluid flow will be described below by way of example as an air flow.
The invention directs electromagnetic energy with respect to the web. The electromagnetic energy may be in the radio frequency (RF) spectrum or wavelength range. If desired, the electromagnetic energy may be in another range, such as that of microwave energy. Reference herein to RF energy includes all such electromagnetic energy capable of contributing to drying or curing as is described herein. Additionally, the electromagnetic energy may be directed to the web as a stray field, through field or both.
With the foregoing in mind, then, it would be desirable to increase the speed of the apparatus and process for drying a web to increase the web throughput while avoiding damage, such as that due to blistering. It also would be desirable to be able to optimize the travel speed of a web in a dryer to reduce time spent in the dryer or in drying the web and to reduce the energy required to dry the web. It also would be desirable to be able to detect conditions related to the drying of a web to achieve the foregoing to facilitate accommodating webs and/or coatings of different materials, size or other parameters, etc.
Conventional air floatation dryers use heated air both to heat the web and/or coating and to remove moisture emitted by the web and/or coating; thus, prior dryers use the heated air to provide both heat transfer and mass transfer. The present invention uses RF energy for heating and can use the air flow for mass transfer or for both heat transfer and mass transfer.